This invention relates to magnetic recording media, and more particularly, to magnetic recording media of coating type using radiation-curable or polymerizable binder.
Several decades have passed since the appearance of magnetic recording media comprising a nonmagnetic substrate and a magnetic layer primarily formed from a magnetic powder of oxide type such as gamma-Fe.sub.2 O.sub.3, gamma-Fe.sub.3 O.sub.4 and cobalt-impregnated gamma-Fe.sub.2 O.sub.3 and a binder. Recently, for the purpose of further improving recording density, magnetic recording media comprising a magnetic layer formed from a ferromagnetic powder such as Fe, Co, Ni, Fe-Co, Co-Ni, Fe-Co-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe-Co-V, etc. and a binder were developed and is now of great interest in the art.
The prior art used thermosetting binders for binding such ferromagnetic fine particles into a magnetic layer. The use of thermosetting binders, however, is rather limited in accomplishing a more efficient and automatic production line as well as increasing the recording density of magnetic recording media.
To overcome the drawbacks of thermosetting binders, Japanese Patent Application Kokai No. 56-124119 proposes a magnetic recording medium having a magnetic layer of magnetic particles bound in a binder capable of curing or polymerizing upon exposure to radiation. Reliable high performance magnetic recording tapes having excellent electromagnetic properties and mechanical properties can be made by combining a binder capable of curing or polymerizing upon exposure to radiation with cobaltmodified needle iron oxide useful in high density recording applications and optionally, needle alloy fine particles having a high coercive force. These tapes find their use in high bias hi-fi audio cassette tape, video cassette tape, and video tape contact-transfer-printing master tape.
These magnetic recording media, particularly for use as magnetic tape and magnetic discs, must fulfil a number of requirements including low dynamic coefficient of friction, smooth and stable travel performance for a prolonged period, improved wear resistance, stability under storage environment to ensure consistent reproduction, and durability (durability of tape both during normal operation and in the still mode). These problems will also be encountered in magnetic recording media having a magnetic recording layer using a radiation-curable or polymerizable binder.
A variety of pre-treatments have heretofore been made on various base films for the purpose of improving durability. Such pre-treatments include treatments with chemical solution, coating, corona discharge treatment, and the like. Wet treatments like chemical solution treatments and coating applied to base films as pre-treatments give rise to a problem when a magnetic composition of magnetic fine particles dispersed in a radiation-curable or polymerizable binder is subseuqently applied to the base film. The binder can interact with the wet treating agent used.
The corona discharge treatment is advantageous because of dry nature eliminating the need for additional steps of rinsing, drying, and disposal of spent liquid. Corona treatment has been carried out for many years and is effective in improving adhesion, wettability, and printability. The corona treatment, however, is not succesful in improving the properties of magnetic recording media to such an extent as to fulfill the high performance which is imposed on the present day and future magnetic recording media.
Another technique known in the art is a flame treatment which is difficult to apply to magnetic recording media which reguire a high degree of dimensional stability.
Under these circumastances, a proposal is made to treat base films with a plasma. The plasma treatment is a one-step dry process and thus has the advantage that drying and disposal of spent solution are unnecessary and no extra material like binders is consumed. In addition, the plasma treatment enables high speed, continuous production so that it can be readily incorporated in the process of manufacturing magnetic recording media without sacrifying production speed and yield.
One technique for plasma treatment of base films is disclosed in Japanese Patent Publication No. 57-42889 (published on Sept. 11, 1982) wherein a treatment is effected with a plasma having a frequency in the range of radio frequency to microwave using a treating gas of air, oxygen, nitrogen, hydrogen, helium, argon, etc. The radio frequency of 13.56 MHz is only described in this publication.
Also, Japanese Patent Application Kokai No. 58-77030 (laid open on May 10, 1983) descloses a process of plasma treatment by applying an AC current at the commercial frequency between electrodes using a treating gas of oxygen, argon, helium, neon or nitrogen. These plasma treatments are somewhat successful in improving the adhesion of a treated base film to a magnetic layer and hence, the durability of magnetic recording media, but not fully satisfactory in bond strength and durability.
Bond strength and durability are still insufficient for magnetic layers using radiation-curable or polymerizable binders.